Node emulator

ABSTRACT

System, methods and apparatii are provided for emulating the performance effect on network traffic flow traversing a node in a communications network. According to one illustrative embodiment, a method of emulating the performance effect on network traffic through a node is provided that includes generating foreground traffic through the node; simulating background traffic at the node; determining an effect of the background traffic on the foreground traffic; and making a forwarding decision with respect to the foreground traffic based on the effect.

BACKGROUND

Event driven “simulation,” a type of computer-based modeling, has become a key tool in the design of data communication networks. Simulation is preferred because of the expense and complexity of constructing a physical model to determine the manner in which a proposed network behaves under certain stimulus. This in turn allows one to discern how its components should be constructed and configured.

For example, prior to purchasing an internet protocol (IP) node (the term “node” refers to, for example, a switch or router) to connect a LAN to the Internet, it is necessary to estimate the bandwidth required for the ports of the IP node or else risk purchasing hardware and bandwidth capacity unsuitable for its intended purpose. It is also useful to study the impact of adding additional load (e.g., increased traffic as a result of additional users, applications, etc.) to an existing network prior to actually obtaining or connecting the resources that would cause the extra load.

Accordingly, use of network simulation applications (also referred to herein as “simulators”) such as Opnet™ (from Opnet Technologies) and Network Simulator (“ns”) are widespread. Such network simulator applications are software-based and permit a user to configure and model an entire network. Thus, on a single personal computer, using a simulation application, a user can simulate the construction of an entire network, determine the viability of particular configurations, evaluate bandwidth/buffer resource allocations, and, in general, compare design options. A simulation application also enables the user to generate traffic patterns, monitor traffic flow, congestion, and discards (if any) at all nodes (or at least all those of interest to a particular analysis) in the simulated network. A “discard” of a packet occurs when a node experiences an overflow condition and fails to forward the packet, for example, by overwriting the packet with an incoming packet. Based on the simulation, a user can make decisions regarding construction of an actual physical network, as well as the configuration of a particular node in terms of necessary bandwidth, queue size, port speed, link capacity, etc., without ever having to construct a physical model.

Network simulation applications, however, generally do not run in real time, and event-driven simulators in particular are designed to model the passage of every packet through the simulated network. Accordingly, simulation applications, due to the increasing network complexity and high speed data transfer rates, require extensive CPU cycles to capture functional details (e.g. traffic characteristics and network element behaviors).

Network emulators, on the other hand, generally perform an abstracted network behavior or function in real-time while typically interfaced to and working in cooperation with actual operational network equipment. This is generally achieved by implementing some of the emulator functionality in hardware and/or embedded firmware in a manner such that it can interface and function as a “black box” that performs its intended tasks in real-time, while some other aspects might be abstracted and modeled internally to create the desired external effect.

SUMMARY

Systems, methods, and apparatus are provided for emulating the performance effect on network traffic flow traversing a node in a communications network. According to one illustrative embodiment, a method of emulating the performance effect on network traffic through a node is provided that includes generating foreground traffic through the node; simulating background traffic at the node; determining an effect of the background traffic on the foreground traffic; and making a forwarding decision with respect to the foreground traffic based on the effect.

According to another illustrative embodiment, a node emulator is provided that includes an ingress media interface, a packet queue to receive incoming foreground traffic from the ingress media interface, an egress media interface, a background traffic generator to simulate background traffic at the node emulator; and a foreground packet scheduler forwarder to cause foreground traffic to be forwarded from the packet queue to the egress media interface.

According to another illustrative embodiment, a system to simulate the effect of background traffic on foreground traffic is provided that includes a client to generate foreground traffic, a node emulator coupled to the client to receive and forward the foreground traffic and to simulate background traffic received by the node emulator, and a server coupled to the node to receive foreground traffic forwarded from the node emulator.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments consistent with the invention and together with the description, serve to explain the principles consistent with the invention.

FIG. 1 is a schematic diagram illustrating a system in which a node emulator is connected in accordance with one illustrative embodiment;

FIG. 2 is a schematic diagram illustrating node architecture modeled by the node emulator of FIG. 1;

FIG. 3 is an architectural block diagram of the node emulator of FIG. 1;

FIGS. 4A and 4B are a flow chart of the scheduler process performed by the node emulator of FIG. 1;

FIG. 5 is a flow chart of the forwarding process performed by the node emulator of FIG. 1; and

FIG. 6 is a flow chart of the background traffic generation process performed by the node emulator of FIG. 1.

DETAILED DESCRIPTION

Reference will now be made in detail to several illustrative embodiments consistent with the present invention, examples of which are shown in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

In accordance with certain illustrative embodiments consistent with the present invention, systems, methods and apparatii are provided for emulating the performance effect on network traffic flow traversing a node (such as a frame relay, ATM and/or IP router) in a communications network. The term “node” as used herein refers to any network device that receives and forwards data packets (most commonly IP packets), cells, or frames (depending on the transport technology used) from one location to another. It should be noted that the term “data packet” used herein refers to any and all types of units of transmitted data, including, without limitation, data packets, cells, and frames. A physical node (herein referred to as a “node emulator”) is provided in order to pass traffic of interest from a source or client (foreground traffic) and demonstrate the effect that simulated background traffic (which is intended to model all other traffic received at the node) has on the foreground traffic.

In one illustrative embodiment, the node emulator models the queuing dynamics at an egress port of the node emulator by simulating background traffic in real time—internal to the node emulator—and making forwarding decisions with respect to the foreground traffic dependent on the background traffic. The node emulator minimizes the number of computations used to simulate high utilization background traffic scenarios by approximating the background process according to a fluid model, thereby limiting per-packet computations to foreground packets. The background traffic can be modeled as a time-varying rate process that is updated periodically.

FIGS. 1-6 show illustrative embodiments of a system, method, and apparatus consistent with principles of the present invention. The system, shown in FIG. 1, is a communications network utilizing a node emulator 10 which is physically connected to a client 20 and a server 30. Client 20 and server 30 are connected to node emulator 10 via, for example, bidirectional Fast Ethernet links 24 and 25, although other connection media can be used. Thus, node emulator 10 need include only two bi-directional ports coupled to links 24 and 25 to support the simple pass through configuration illustrated in FIG. 1.

Client 20 communicates to server 30 via node emulator 10 by, for example, issuing application transaction requests that are in turn transmitted in the form of data packets. Server 30 responds by issuing corresponding application transaction responses. The traffic passing to and/or from client 20 and server 30 via node emulator 10 is herein referred to as “foreground traffic.” Node emulator 10 internally simulates the generation of other contending traffic sources, herein referred to as “background traffic,” to determine the performance impact of the background traffic on the foreground traffic.

Foreground traffic can be generated, for example, by test software, such as Chariot™, which is a commercially available software application from NetIQ for generating application traffic. Thus, in the foregoing illustrative embodiment, test software such as Chariot™ would be installed in client 20 and would be generating traffic sent to server 30 via node emulator 10. In general, the foreground traffic can be generated by any pair of hosts running peer-to-peer (e.g., Voice-over-IP, gaming, etc.), client/server (e.g., web browsing, database query, FTP, etc.), or any other real application between two endpoints. Thus, foreground traffic consists of actual data packets generated in real time by client 20 and server 30, while simulated background traffic is generated internal to emulator 10 (as explained below).

Node emulator 10, physically similar to an actual node, can be configured in terms of bandwidth, port speed, queue size, queuing policy, etc., to model the egress port functionality of a node. Regarding control plane (e.g., signaling) and routing protocol support, the node emulator can be implemented as “transparent” or “protocol aware.”

The node emulator 10 can be implemented as a “transparent” (i.e., protocol-unaware) device that simply forwards or discards packets (e.g., IP packets), frames (e.g., Ethernet, Frame Relay, POS frames), or cells (e.g., ATM cells) with no modification to any of the packet, frame, or cell headers. Only two ports of the same media type are supported in this mode (as depicted in FIG. 1).

Alternatively, the node emulator 10 can operate as a “protocol-aware” device that performs switching and/or routing control plane operations and associated packet, frame, or cell header translations. In this case, node emulator 10 appears as a fully operational switch or router to the adjoining client, server, or other real nodes. Node emulator 10 performs routing (e.g., for IP router emulation), signaling (e.g., for ATM switch emulation), and header field modifications in a manner identical to a real node between two ports (as depicted in FIG. 1). (Additional “real” ports can be added and emulated as desired, with a resultant increase in complexity.)

For instance, in the case of IP node emulation, node emulator 10 performs address resolution protocol (ARP) functions, constructs a routing table, performs the TTL decrement and check sum calculation, etc., for foreground traffic IP packets forwarded through the node represented by emulator 10. In this case, node emulator 10 is a fully functional, multi-port, switch or router having the additional capability of internally generating background traffic and subjecting incoming foreground traffic to queueing level impairments as a result of that background traffic. It should be noted that when functioning as “protocol-aware” IP router, emulator 10 further supports link-level media translations (e.g., IP-over-ATM ingress port to IP-over-Ethernet egress port) occurring between client 20 and server 30. A dedicated microprocessor can be included in emulator 10, as discussed below with respect to FIG. 3, to support the foregoing “protocol-aware” control functions.

When node emulator 10 is implemented in the “transparent” mode, operation differs depending on whether the node is emulating an IP node or a frame level node. When a packet layer (IP layer or other network or OSI layer 3) node (such as an IP router) is chosen to be emulated, emulator 10 performs IP (or other network layer) packet de-encapsulation (at ingress) and encapsulation (at egress) so that impairment operations (e.g., foreground packet discards and delays) can be performed at the IP packet level. If so, simplicity (of transparent mode) is maintained by encapsulating the packets to be forwarded (upon egress) in identical copies of the link layer frames in which those packets arrived.

Alternatively, if the node is “transparent” and emulating a frame level node, a frame is forwarded without de-encapsulation, encapsulation, or header modification. Other than this distinction in operation between transparent modes for packet layer nodes vs. frame layer nodes, there is no significant distinction in the “transparent” mode implementation in terms of its primary functions. For that reason, packet level operation is assumed unless specified otherwise, for the balance of this application, without loss of generality. That is, the data units traversing the emulator node will be referred to as data packets (which include cells, frames, or network layer packets). A “transparent” mode implementation is assumed unless specified otherwise for the balance of this application.

The type and volume of background traffic generated internal to node emulator 10 is also configurable. A graphical user interface (GUI) associated with node emulator 10 enables a user to configure the node emulator. Thus, depending on the configuration of the node and the background traffic, node emulator 10 will make a forwarding decision with respect to each foreground traffic data packet received from client 10 and server 30. The forwarding decisions include: forwarding the data packet, delaying and forwarding the data packet, or discarding the data packet. As such, the end-to-end (e.g., client to server) response time, throughput, delay, lost data, and other performance characteristics can be measured at client 10 and/or server 30. Moreover, some packet level statistics (e.g., packet delay, packet delay variation, packet loss) of the foreground data flow can be collected at the node emulator itself.

Node emulator 10 models node level network impairments incurred by a foreground traffic flow to and from client 20 as if that foreground traffic flow was contending with other coincident background traffic. FIG. 2 illustrates an example of the multiplexing scenario modeled (or emulated) by node emulator 10.

In FIG. 2, only client 20 and sever 30 are true physical hardware, and links 24 and 25 are the only true physical links connecting client 20 and server 30 externally to node emulator 10. Emulator 10 internally models the aggregate effect of simulated background clients 50, 60, . . . N, simulated switch fabric 80, and simulated queues 90, 110, and 120 (with associated simulated links 125, 127, and 128) on foreground traffic to/from client 20 and server 30. As explained hereinafter, hardware or software operating on node emulator 10 is used to emulate the node level impairments resulting from multiple sources contending for the same port resources as illustrated in FIG. 2. Generally, hardware, firmware, or software implements an algorithm adapted to calculate, among other things, how long the foreground traffic should be delayed (to represent queuing of the foreground traffic into queue 100), or whether to discard the foreground traffic due to an overflow. Note that (for each direction) node emulator 10 only models one of the ports of a true node (each link 24 and 25 is connected to a physical port of node emulator 10).

In an actual node, switch fabric 80 depicted in FIG. 2 would normally provide some small additional delay (in addition to any queueing delays experienced by packets), and is modeled by providing some small additional fixed delay to all packets that are forwarded (i.e., not discarded) by node emulator 10. Since this delay is usually negligible relative to the queueing delays, it is not accounted for in the foregoing illustrative embodiment. However, if desired, the fixed delay is incorporated by adding a small additional constant value to the waiting time experienced by those packets that are forwarded through (i.e., not discarded) node emulator 10.

FIG. 3 shows an architectural block diagram of node emulator 10 used to model traffic flow in one direction, e.g., from client 20 to server 30. The architecture of FIG. 3 is replicated (once for each direction) to support bi-directional operation as shown in FIG. 1. An ingress media interface 150 provides the physical media receiver and MAC layer functions used for interfacing the foreground traffic flow from client 20 to node emulator 10. That is, interface 150 provides physical and link layer termination for terminating the Layer 2 framing structure used to encapsulate the foreground traffic flow, according to the particular transport technology being utilized.

An egress media interface 160 provides link layer encapsulation and physical layer transmission functions analogous to ingress media interface 150. Thus, any commonly deployed LAN or WAN media (e.g., Ethernet, Frame Relay, ATM, POS, etc.) can be supported by node emulator 10. Although the foregoing illustrative embodiment utilizes Fast Ethernet, other possible interfaces, without limitation, include: Frame Relay/PPP transceiver and framing logic; ATM transceiver logic with AAL5 Segmentation and Reassembly (SAR); pure ATM cell processing without SAR, Packet over SONET (POS); and others. Thus, ingress and egress media interfaces 150 and 160 interface the particular transport technology being used with the queuing simulation functions performed by the node emulator 10 explained hereinafter.

A foreground packet scheduler forwarder (FPSF) 170 controls the transfer of an incoming foreground packet from ingress media interface 150 to a packet queue 180 to egress media interface 160. In the foregoing illustrative embodiment, the node emulator architecture includes background traffic generators 190 and 200, associated latches 195 and 205, a time slot pulse generator 210 (in this case a divide by r counter), a system clock 215, and a time stamp module 220.

FPSF 170 can be a hardware logic implementation (e.g., implemented in VHDL), a software application (e.g., written in C or C++), or a firmware application operating in node emulator 10, although the invention is not limited to any particular hardware, firmware, or software implementation or language. Generally, FPSF 170 performs computations to determine whether a foreground traffic data packet received at ingress media interface 150 from client 20 should be discarded (e.g., to model a queue overflow event), and, if the packet is not to be discarded, FPSF schedules a time at which the data packet is to be forwarded from the packet queue memory 180 to the egress media interface 160. The scheduled time is calculated to simulate the time in which the foreground data packet would be waiting in the particular queue modeled (e.g., queue 100) in FIG. 2.

When the scheduled time for forwarding is reached for a data packet, a forwarding process causes the data packet to be forwarded from packet queue 180 to egress media interface 160, which in turn encapsulates the contents of the data packet in a frame structure according to the particular media being used (e.g., Fast Ethernet in FIG. 1), performs the appropriate header field modifications (e.g., TTL decrement for IP), and transmits the packet onto link 25. The time in which the data packet waits in packet queue 180 for its scheduled forwarding time is referred to as the waiting time for that particular data packet. The sample distribution (e.g., histogram) and average value of waiting times represent examples of statistics that can be monitored and logged by the node emulator.

Thus, the primary functions of FPSF 170 are first to make a packet discard decision and then, if the packet is not discarded, to compute a waiting time for each arriving foreground traffic data packet during which the data packet remains in queue 180. The waiting time for a given data packet is a function of the queuing policy being simulated (which can be selected by a user via a GUI). According to the foregoing illustrative embodiment, one of two queuing policies can be selected (1) FIFO or (2) Static Priority, although other queuing policies can be implemented (e.g., WFQ). Generally, as discussed in detail below, when the Static Priority queueing policy is selected, node emulator 10 can also be configured to internally model background traffic having higher priority than the foreground traffic generated by client 20, thereby emulating the effect that such traffic would have on the foreground traffic.

FIGS. 4A and 4B show a flow diagram illustrating the scheduling process executed by FPFS 170. The following variables and functions are used herein to facilitate an understanding of the computations executed by FPFS 170 in accordance with the foregoing illustrative embodiment:

-   -   j=packet index incremented upon arrival of each data packet;     -   pktlen[j]=size of the jth packet (in bytes);     -   Tin(j)=arrival time of the jth arriving packet (in seconds) as         captured by the timestamp module;     -   Tout[j]=time scheduled for the jth packet to be forwarded to         egress media interface 160;     -   W[j]=waiting time for the jth packet to be forwarded to the         egress media interface (in seconds);     -   k=time slot index incremented at the beginning of each time         slot;     -   T0=length of each time slot (in seconds);     -   Tslot[k]=time at the beginning of the kth time slot as captured         by the timestamp module. Theoretically, Tslot[k]=k*T0 and thus         should always approximate that value;     -   R[k]=background traffic volume (in bytes per second) generated         during the kth time slot;     -   R1[k]=volume of high priority background traffic generated         during the kth time slot (bytes per second);     -   Qlen=occupancy of the simulated queue. In general, Qlen is         updated immediately following either a “packet arrival” event or         “beginning of time slot” event. All foreground packets flow         through this queue;     -   Qlen1=occupancy of the simulated higher priority queue (when         static priority queueing is selected). This variable is not used         when FIFO queueing is selected. Note that only simulated high         priority background traffic effects this queue; foreground         traffic does not flow through this queue;     -   last_event_time=temporary variable used in Qlen update         calculations. It represents the time of the last event (either a         “packet arrival” or “beginning of timeslot” event) prior to the         time this variable is calculated;     -   temp_Qlen=temporary variable used in Qlen update calculations;     -   temp_Qlen1=temporary variable used in Qlen1 update calculations;     -   d=additional waiting time delay caused by additional high         priority traffic that arrives before the packets current waiting         time expires (i.e. accounts for pre-emptive effect of higher         priority traffic in when Static Priority queueing is selected).     -   C=output (or egress) link capacity being simulated by the node         10 (FIG. 1) (in bits per second). Note that C may be selected to         be a value less than the actual egress port capacity of the node         emulator to model a lower speed link. The default value is the         bit rate capacity of the egress port so a Fast Ethernet port,         for instance, would have a default value of 100 Mb/s (though C         could be provisioned to a lower value);     -   a[k]=rate of growth of the simulated queue occupancy (in bytes         per second) for the duration of the kth timeslot. Thus, a[k] is         the rate of growth (or derivative) of Qlen during time period         Tslot[k]<t<Tslot[k+1];     -   a1[k]=Rate of growth of the simulated high priority queue         occupancy (in bytes per second) for the duration of the kth         timeslot when Static Priority queueing is in effect, i.e., a1[k]         is the rate of growth of Qlen1 during the time period         Tslot[k]<t<Tslot[k+1];     -   Qmax=maximum capacity of the simulated queue. A packet arrival         that would cause Qlen to exceed Qmax is discarded;     -   Qmax1=maximum capacity of the simulated high priority queue.         Thus, Qlen1 is upper bounded by this value;     -   l=cumulative count of discarded packets. This can be used to         calculate the foreground packet loss ratio according to l/j;     -   Tot_bytes_gen=cumulative count of the total bytes generated by         the lower priority background traffic generator plus total         number of bytes resulting from all foreground packets that have         arrived;     -   Tot_bytes_gen1=cumulative count of the total bytes generated by         the higher priority background traffic generator;     -   Tot_bytes_lost1=cumulative count of total lower priority bytes         lost (includes lost bytes from lower priority background         generator and cumulative total bytes of discarded foreground         packets); and     -   Tot_bytes_lost1=cumulative count of total higher priority bytes         lost (includes lost bytes from higher priority background         generator)

In step 300 of FIG. 4A, FPSF 170 initializes the system in which

j=k=l=0;

Tot_bytes_gen=Tot_bytes_lost=0;

Tot_bytes_gen1=Tot_bytes_lost1=0;

Tin[0]=Tout[0]=R[0]=R1[0]=W[0]=0; and

Qlen=Qlen1=0.

FPSF 170 (FIG. 3) detects either the arrival of a data packet at ingress media interface 150 via packet arrival signal 155 from ingress media interface, or a beginning of time slot signal from timeslot generator 210 (step 310). Upon receipt of the packet arrival signal, in step 320, FPSF 170 performs the following functions: packet counter j is incremented, the arrival time of packet j is recorded (Tin[j]=Tstamp(pkt[j])), the packet length (in bytes) is determined (pklen[j]=get_pktlen(pkt[j])), the simulated (lower priority) queue length (Qlen) is updated (detailed below), the total bytes received counter (Tot_bytes_gen) is updated (detailed below), and the packet is transferred from ingress media interface 150 to queue 180. Time stamp module 220 (FIG. 3) is used to timestamp the arrival of each packet and timestamp the beginning of each time slot, which are denoted as Tin[j] and Tslot[k], respectively.

The simulated queue length calculation (in step 320) and Tot_bytes_gen counter update, are performed as follows. Assuming that the jth packet arrival occurs during time slot k, i.e., Tslot[k] is less than Tin[j] and Tin[j] is less than Tslot[k+1], the simulated queue length immediately after the jth packet arrives can be calculated using the relation: temp_(—) Qlen=Qlen+pktlen[j]+a[k]*(Tin[j]−last_event_time)/T0, Qlen=Max(temp_(—) Qlen,0);

where

last_event_time=Max(Tin[j−1], Tslot[k]), and

a[k]=R[k]−(⅛)*C(bytes/sec).

and Tot_bytes_gen counter is updated according to Tot_bytes_(—) gen=Tot_bytes_(—) gen+pktlen[j];

As seen from the foregoing relation, the length of the simulated queue immediately after receiving the jth packet is calculated by taking into account: the simulated queue length prior to receiving the arriving packet, the rate of growth of the queue during the timeslot the packet arrived in (a[k]), the packet length of the arriving packet (pktlen[j]), and the time of the last event prior to the packet arrival (denoted by last_event_time). The rate of growth of the simulated queue, a[k], during timeslot “k” depends on the current value of the sequence R[k] generated by background traffic generator 190.

The function Max(x, y) is defined as the greater of x and y and is used in the above equation to ensure that Qlen has a nonnegative value, since a[k] (the rate of growth of the simulated queue) can have a negative value (and consequently, temp_Qlen can be negative), but the queue occupancy (Qlen) cannot. This can occur when the background traffic rate sequence R[k] is less than the node's simulated link capacity (⅛)*C, resulting in the rate of growth of the simulated queue, a[k], having a negative value indicating a decreasing rate of growth of queue size during the kth time slot.

The temporary variable last_event_time=Max(Tin[j−1], Tslot[k]) is used in the above equation to specify the time of the last event prior to the jth packet arrival. Note that the term “event” applies to either a “packet arrival” or “beginning of timeslot” event. Since it is assumed that the jth packet arrival occurs within the kth timeslot, the last event just prior to that arrival must be either the beginning of the kth time slot (with timestamp Tslot[k]), or the (j−1)th packet arrival (having timestamp Tin[j−1]). The timestamp of the event which occurred the latest amongst those two possibilities is of course Max(Tin[j−1], Tslot[k]).

It should be noted that the simulation becomes less accurate when T0 (length of the time slot k) is chosen too large because the approximation of the actual queue dynamics becomes more coarse as T0 is increased. This is explained by the fact that the background traffic generator only models the background traffic dynamics at intervals of T0 and only approximates (i.e., interpolates) that behavior at times within these intervals according to the linear rate of growth function, a[k]. On the other hand, the computation burden is increased as T0 decreases due to more frequent changing of the background traffic rate R[k]. Thus, there is a design tradeoff in the selection of T0. A preferred design rule of thumb is to select T0 such that Max(a[k]*T0) is only a fraction of, typically on the order of one-half or less, Qmax, the maximum occupancy of the simulated queue before overflow. Note that Max(a[k]*T0) represents that maximum change in occupancy of the simulated queue during any given timeslot. As such, T0 can be selected fairly easily if one knows the upper bound on R[k] (which in turn would bound a[k]), otherwise, T0 can be estimated on the basis of some knowledge of the background traffic behavior.

T0 account for a possible overflow of the simulated queue, the simulated queue length (Qlen) calculated in step 320 is compared, in step 330, to the maximum occupancy of the simulated queue (Qmax). If the calculated simulated queue length is greater than the maximum occupancy of the simulated queue (Qmax), an overflow condition is present, and, in step 340, the packet is discarded (e.g., removed from the queue 180 via pointer manipulation, or, alternatively, overwritten upon the next packet arrival), the packet loss counter l is incremented by one, i.e. l=l+1, the total bytes lost statistic variable (Tot_bytes_lost) is incremented by the byte length of the discarded packet (Tot_bytes_lost=Tot_bytes_lost+pktlen[j]), and the queue length is recalculated by subtracting the length of the discarded packet from the queue length (Qlen=Qlen−pktlen[j]). The process then returns to 310 to wait for the occurrence of the next “packet arrival” or “start of next timeslot” event.

If an overflow condition is not encountered in step 330, then the packet is not discarded and the waiting time for the data packet to be forwarded to egress media interface 160 is computed. The waiting time (W[j]) simulates the time that the jth arriving data packet from client 20 would wait in queue 100 of FIG. 2. Note that if the jth arriving packet encounters an overflow condition, then W[j] is not calculated for that packet since it will be discarded in step 340. In the foregoing illustrative embodiment, FPSF 170 uses one of two different waiting time calculation methods depending on whether node emulator 10 is configured for a FIFO or Static Priority queue policy (step 350).

If the node emulator is configured to model a FIFO queueing policy, then, in step 360, the waiting time for the packet is calculated by the relation: W[j]=(Qlen*8)/C which takes into account the current simulated queue occupancy (Qlen) and the link capacity (C) of the emulated node.

If node emulator 10 is configured to model a Static Priority queueing policy, then in step 370, the waiting time for the packet is calculated by W[j]=(Qlen+Qlen1)*(8)/C When node emulator is configured for Static Priority queueing, Qlen represents the length (or occupancy) of the lower priority queue (which includes the foreground traffic from client 20), and Qlen1 represents the length of the high priority queue. Therefore, the above relation takes into account both the lower priority queue occupancy (Qlen), higher priority queue occupancy (Qlen1), and the link capacity (C) of the node.

In step 380, after performing the waiting time calculation, in step 360 or 370, the packet is then scheduled to be forwarded to the egress media interface 160 by FPSF 170 by summing the waiting time W[j] calculated in step 360 or 370 with the time the packet was received (Tout[j]=Tin[j]+W[j]). Tout[j] is then placed on a forwarding “schedule list” (to be subsequently read by the FPSF 170 when time Tout[j] is reached (see FIG. 5). Thus, FPSF 170 calculates the waiting time for each incoming packet from client 20, which is intended to reflect the actual waiting time the data packet would encounter if the client was connected to an actual networked node.

After the current data packet is either discarded (step 340), or scheduled for forwarding (step 380), the process repeats by returning to 310.

If, in step 310, a Start of Next Time Slot is detected by FPSF 170 (as opposed to a packet arrival), then, in step 311, first the timeslot counter “k” is incremented by one, and the simulated queue length (Qlen) is updated according to: temp_(—) Qlen=Max(Qlen+a[k−1]*(Tslot[k]−last_event_time)/T0,0), Qlen=Min(temp_(—) Qlen,Qmax),

where

last_event_time=Max(Tin[j], Tslot[k−1]), and

a[k−1]=R[k−1]−(⅛)*C(bytes/sec),

and the total bytes generated (Tot_bytes_gen) and total bytes lost (Tot_bytes_lost) counters are updated according to: Tot_bytes_(—) gen=Tot_bytes_(—) gen+R[k−1]*T0, Tot_bytes_lost=Tot_bytes_lost+Max(temp_(—) Qlen−Qmax,0). In step 312, the new background traffic volume sequence value R[k] (bytes/sec) for the updated timeslot index value “k” is read from latch 195.

The above Qlen update calculation in step 311 is performed upon a “start of next timeslot” event indication signal (from timeslot pulse generator 210, FIG. 3). Since the last packet arrival occurred at time Tin[j] (based on the current value of index j at the time the “beginning of timeslot” event occurred), the last_event time occurred at either time Tslot[k−1] or time Tin[j], whichever of these happened latest, i.e., last_event_time=Max(Tin[j], Tslot[k−1]). Also, since the timeslot index “k” is incremented by one at the beginning of step 311, a[k−1] is used as the rate of growth for Qlen between the last_event_time and time Tslot[k] to calculate the final occupancy of the simulated queue at the end of previous timeslot (which is equivalent to the occupancy upon the occurrence of the current “start of next timeslot” event).

After step 312, if node emulator 10 is configured to provide FIFO queueing (as determined in decision step 313), then FPFS 170 (FIG. 3) returns to step 310 and waits for the next event. If the node emulator is configured to provide Static Priority queueing, FPSF 170 reads the high priority traffic volume sequence value, R1[k](bytes/sec), from high priority background traffic generator latch 205 (step 314), using the updated timeslot index “k” (updated in step 311).

R1[k], in step 315, is then used by FPSF 170 to update the high priority simulated queue occupancy (Qlen1), according to: temp_(—) Qlen1=Qlen1+Max(a1[k]*T0,0), and Qlen1=Min(temp_(—) Qlen1,Qmax1);

where

a1[k]=R1[k]−(⅛)*C (bytes/sec),

and the high priority total bytes generated and lost counters (Tot_bytes_gen1 and Tot_bytes_lost1) are updated as follows: Tot_bytes_(—) gen1=Tot_bytes_(—) gen1+R1[k]*T0; and Tot_bytes_lost1=Tot_bytes_lost1+Max(temp_(—) Qlen1−Qmax1,0).

If the value of R1[k] read from higher priority background traffic generator latch 205 (in step 314) is greater than zero (as determined in decision step 316), then an additional delay d=(R1[k]*T0*8)/C, is added to all scheduled times Tout[j] currently on the forwarding “schedule list” (from step 380) in step 317 (otherwise the flow returns to step 310). That is, Tout[j]=Tout[j]+d, for all Tout[j] currently in the “schedule list.”

The “schedule list” contains all scheduled forwarding times, Tout[j], previously placed in the list (via step 380), that have yet to be removed from the list by the forwarding process (step 420 of FIG. 5).

The purpose of adding the additional delay, d, is a consequence of the fact that the higher priority traffic pre-empts lower priority traffic and hence the scheduled time to forward a foreground traffic data packet (computed in step 370), becomes invalid if additional high priority traffic were to arrive before the scheduled forwarding time (Tout[j]) for that packet is reached.

The forwarding process executed by FPFS 170 (FIG. 3) is represented in flowchart form in FIG. 5. A data packet in queue 180 is forwarded to egress media interface 160 when the waiting time for that packet has expired, i.e., when time Tout[j] occurs. Note that while index “j” is used by the FPSF scheduling process (FIGS. 4A and 4B), to reference the schedule times (Tout[j]) placed in the schedule list in step 360, and to align those schedule times with the packet arrival index j, a different (and independent) index “i” is used to by the forwarding process (FIG. 5) to reference the schedule times currently in the list. Counter “i” is set to zero in initialization step 400, and is incremented each time a packet is forwarded to egress media interface 160 (FIG. 3) as explained herein. The index “i” is used by the forwarding process to restore continuity in the index values for entries Tout[i] in the “schedule list”. Continuity is lost for the scheduler process index “j,” as any discarded packets will cause the “j” index to lose continuity for packets that are not discarded.

In step 405, the forwarding process performed by FPSF 170 consecutively reads scheduled forwarding times from the schedule list formed in step 360, and waits for the current time to equal the scheduled time (Tout[i]) for the next packet to be forwarded (steps 410-415). When the time equals Tout[i] the ith packet is forwarded to the egress media interface, and then entry Tout[i] is removed from the “schedule list” (step 420).

Since the schedule times are read from the “schedule list” in the same order in which they placed in that list (i.e., in a first-come-first-serve manner), we always are assured to have Tout[i]<Tout[i+1].

Background traffic generator 190 updates the value of the background traffic sequence (R[k]) contained in latch 195 at the beginning of each time slot “k”, in the manner indicated by the flow diagram of FIG. 6. After initialization (step 425), at the end of each time slot of length T0, a counter increments “k” and the next value of R[k] is determined using a serial random number generator, table lookup scheme, or other appropriate sequence generator (step 430). The process then waits T0 seconds for the next time slot to begin (steps 435-440). When the next time slot occurs, in step 445, background traffic generator 190 places the background traffic sequence determined in step 430 in latch 195 (which is subsequently read by FPSF 170 in step 312 as explained above). This process is repeated for each time slot.

Note that the value of the timeslot index “k” for each of FPSF 170 and background traffic generators 190 and 200 will differ (typically by 1 in an ideal case where both processes are initialized coincidentally), but this difference is inconsequential since the FPSF and background traffic generators operate independently (though synchronized to the same timeslot pulse generator 210 and signaled on timeslot events simultaneously). As can be observed in FIG. 6, “k” corresponding to the background generator process is immediately set to 1 after initialization in the background generator process (step 430), but “k” for the FPSP waits until the first timeslot event has occurred in the FPSP process (step 311, FIG. 4A).

Indexes “j” and “i” used above in describing the scheduling (FIG. 4) and forwarding (FIG. 5) processes, respectively, are incremented asynchronously with respect to each other. Index “j” is used to indicate the arrival of the jth packet at node emulator 10, and index “i” is used to indicate the ith packet to be forwarded through the node emulator. Index “k” which denotes the kth time slot for background traffic generation (FIG. 6), and also is used by the FPSF to index the kth timeslot event (FIG. 4), is independent with respect to these two processes, and independent and asynchronous with respect to indexes “i” and “j” as well. Thus each process independently increments its own corresponding index(es) as described above. The three processes (scheduling, forwarding, and background traffic generation) run concurrently and thus can be implemented via concurrent hardware state machines, software threads in a dedicated processor, or other appropriate means.

When node emulator 10 (FIG. 2) is configured to perform a Static Priority queueing policy, it simulates background traffic having higher priority than the foreground traffic generated by client 20, and thereby simulates the effect that such traffic would have on the foreground traffic. In the foregoing illustrative embodiment, an additional background traffic generator 200 (FIG. 3) is used to generate a background traffic sequence having a higher priority than the foreground traffic.

Background traffic generator 200 generates high priority background traffic sequence R1[k] and places a new sequence value in latch 205 for each time slot “k” in a manner analogous to background traffic generator 190 explained above (FIG. 6). Both Background Generators are synchronized to the same time slot pulse generator 210. Thus, latches 195 and 205 are updated at the same time at the beginning of a new time slot k. However, they may, and typically would, have completely independent outcomes. Thus, the sequences R[k] and R1[k] are assumed to be independent. One can configure node emulator 10 to generate background traffic having the same priority as the foreground traffic and background traffic having a higher priority than the foreground traffic, to model the impact of higher priority traffic on the foreground traffic flow from client 20.

The flow diagram of FIG. 6 also applies to background generator 200 with the exception that in step 445, the outcome sequence is denoted by R1[k], and is placed in latch 205 (instead of latch 195).

Node emulator 10 (FIG. 2) enables a determination of the real-time effect of simulated background traffic on actual foreground traffic data packets generated by client 20. The node emulator mimics the packet loss and delay effects on a foreground traffic flow that traverses a single node subject to a quantifiable background traffic load—where the background traffic load is modeled by a periodic sequence of rate update events.

According to one illustrative embodiment disclosed herein, rather than modeling each background packet explicitly, a sequence of background traffic rates, R[k] (in bytes per second), per time interval T0 (seconds), is used to approximate the dynamically varying volume of background traffic. The background traffic sequence is updated after each time slot interval where “k” is the index of the kth time interval. The sequence R[k] is generated internal to the system independent of the foreground traffic packet arrival times and is used to approximate the actual background traffic process according to a fluid approximation, i.e., a steady rate R[k] during the kth time slot. This fluid analogy background traffic modeling approach mitigates the amount of total computations required for modeling a highly utilized switch or router port, as delay or discard computations are only performed for each foreground traffic packet.

The particular background traffic model to be used is user configurable, and can be generated internally via a psuedo random number generator, table lookup per time slot, or other suitable methodology. By use of table lookup, the background traffic process can be pre-computed offline and downloaded to the node emulator to provide a predetermined sequence of R[k] values to be read from memory for each time slot. Thus, any background traffic process can be applied that can be represented in terms of the number of bytes that arrive over a time interval of length T0. Some examples are Poisson distribution, Gaussian distribution, and Self-Similar (fractal) distribution models.

It will be apparent to those skilled in the art that additional various modifications and variations can be made consistent with the present invention without departing from the scope or spirit of the invention. For example, if it is desired to simulate the queuing effects of a given background traffic load alone without emulating foreground traffic, one could use the tot_bytes gen and tot_bytes_lost statistics to examine the behavior of the node emulator subject to that background traffic. In this case, the node emulator would function solely to provide queuing simulation information in real time.

Other embodiments consistent with the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. 

1. A method of emulating network traffic through a node having connections to a server and a client, comprising: (a) generating foreground traffic through the node from the server to the client, wherein the foreground traffic comprises foreground data packets; (b) simulating background traffic internally at the node, the simulated background traffic contending with the foreground traffic at the node; (c) determining, at the node, an effect of the simulated background traffic on the foreground traffic received at the node, wherein the determining comprises simulating a queue based on at least the simulated background traffic and the foreground traffic received at the node, wherein the determining further comprises: computing a rate of growth of the simulated queue based on at least the simulated background traffic and an output link capacity associated with the node; and calculating a length of the simulated queue based on at least the computed rate of growth and lengths of the foreground data packets; and (d) making a forwarding decision at the node with respect to the foreground traffic based on the calculated length of the simulated queue, wherein the forwarding decision comprises one of: (i) forwarding at least one of the foreground data packets through the node via an egress media interface to the connection to the client; or discarding at least one of the foreground data packets.
 2. The method of claim 1, wherein the computing comprises computing the rate of growth of the simulated queue according to: a[k]=R[k]−(1/8)*C, wherein: a[k] represents the rate of growth of the simulated queue (in bytes per second) for a duration of a kth timeslot; R[k] represents a background traffic volume (in bytes per second) generated during the kth time slot; and C represents the output link capacity of the node (in bits per second).
 3. The method of claim 1, wherein the forwarding decision comprises one of: scheduling the foreground data packet to be forwarded through the node, forwarding the foreground data packet through the node, and discarding the foreground data packet.
 4. The method of claim 1, wherein the forwarding decision includes delaying forwarding the foreground data packet for a period of time.
 5. The method of claim 4, wherein the forwarding decision includes calculating the period of time.
 6. The method of claim 5, wherein the period of time simulates the length of time the foreground data packet remains in the simulated queue before being forwarded from the node.
 7. The method of claim 1, wherein the forwarding decision includes discarding the foreground data packet when it is determined that the simulated background traffic and foreground traffic would cause the simulated queue to overflow.
 8. The method of claim 1, wherein step (b) includes simulating a rate of background traffic.
 9. The method of claim 8, wherein simulating the rate of background traffic comprises simulating the background traffic at a rate of a number of bytes per second.
 10. The method of claim 9, wherein the number of bytes per second is randomly selected.
 11. The method of claim 10, wherein the simulated rate is assigned via a lookup table.
 12. The method of claim 1, wherein the foreground traffic has a first priority and wherein step (b) includes assigning the simulated background traffic a second priority that is different from the first priority.
 13. The method of claim 1, wherein the length of the simulated queue includes at least the length of an incoming foreground data packet plus the length of the simulated queue just prior to the receipt of the incoming foreground data packet.
 14. A node emulator, having connections to a server and a client, comprising: an ingress media interface, which receives incoming foreground traffic from the connection to the server; a packet queue to receive incoming foreground traffic from the ingress media interface, wherein the foreground traffic comprises foreground data packets; an egress media interface, which transmits foreground traffic to the connection to the client; a background traffic generator to simulate background traffic internally at the node emulator, the simulated background traffic contending with the foreground traffic at the node emulator; and a foreground packet scheduler forwarder to cause the foreground data packets to be forwarded from the packet queue to the egress media interface, wherein the foreground packet scheduler forwarder: simulates the packet queue based on at least the simulated background traffic and the foreground traffic; computes a rate of growth of the simulated packet queue based on at least the simulated background traffic and an output link capacity associated with the node emulator; calculates a length of the simulated packet queue based on at least the computed rate of growth and lengths of the foreground data packets; and makes a forwarding decision with respect to foreground traffic in the packet queue, the forwarding decision being dependent on the calculated length of the simulated packet queue, and wherein the forwarding decision comprises one of: (i) forwarding a foreground data packet through the node emulator to the egress media interface; or (ii) discarding the foreground data packet.
 15. The node emulator of claim 14, wherein the foreground packet scheduler forwarder computes the rate of growth of the simulated queue according to: a[k]=R[k] −(1/8)*C, wherein: a[k] represents the rate of growth of the simulated queue (in bytes per second) for the duration of a kth timeslot; R[k] represents a background traffic volume (in bytes per second) generated during the kth time slot; and C represents the output link capacity of the node (in bits per second).
 16. The node emulator of claim 14, wherein the forwarding decision includes delaying forwarding the data packet for a period of time.
 17. The node emulator of claim 16, wherein the forwarding decision includes calculating the period of time.
 18. The node emulator of claim 17, wherein the period of time simulates the length of time the data packet remains in the simulated queue before being forwarded from the node.
 19. The node emulator of claim 14, wherein the forwarding decision includes discarding the foreground data packet when it is determined that the simulated background traffic and foreground traffic would cause the simulated queue to overflow.
 20. The node emulator of claim 14, wherein the foreground packet scheduler forwarder is a software application running on the node emulator.
 21. The node emulator of claim 14, wherein the forwarding decision comprises one of: forwarding the foreground data packet through the node to the egress media interface, discarding the foreground data packet, and scheduling the foreground data packet for forwarding to the egress media interface.
 22. The node emulator of claim 14, wherein the scheduling is dependent at least in part on an amount of the simulated background traffic.
 23. A system to emulate the effect of simulated background traffic on foreground traffic comprising: a server to generate foreground traffic, wherein the foreground traffic includes foreground data packets; a node emulator coupled to the server and configured to receive the foreground traffic, to simulate background traffic to contend with the foreground traffic at the node, and to forward the foreground traffic via an egress media interface; and a client coupled to the node emulator to receive the foreground traffic forwarded from the node emulator via the egress media interface, wherein the node emulator: simulates a queue based on at least the simulated background traffic and the foreground traffic received at the node emulator; computes a rate of growth of the simulated queue based on at least the simulated background traffic and an output link capacity associated with the node emulator: calculates a length of the simulated queue based on at least the computed rate of growth and lengths of the foreground data packets; and makes a forwarding decision with respect to the foreground traffic dependent on the computed length of the simulated queue, and wherein the forwarding decision includes one of: (i) discarding a foreground data packet; or (ii) scheduling the foreground data packet for forwarding to the client.
 24. The system of claim 23, wherein the node emulator computes the rate of growth of the simulated queue according to: a[k]=R[k] −(1/8)*C, wherein: a[k] represents the rate of growth of the simulated queue (in bytes per second) for the duration of a kth timeslot; R[k] represents a background traffic volume (in bytes per second) generated during the kth time slot; and C represents the output link capacity of the node (in bits per second).
 25. The system of claim 23, wherein the forwarding decision includes one of: discarding a foreground data packet, forwarding the foreground data packet to the client, or scheduling the foreground data packet for forwarding to the client.
 26. A computer readable medium containing instructions for performing a method of emulating network traffic through a node having connections to a server and a client, the method comprising: (a) simulating background traffic internally at the node, the simulated background traffic contending with foreground traffic received at the node; (b) determining, at the node, an effect of the simulated background traffic on the foreground traffic received at the node, wherein the foreground traffic comprises foreground data packets received at the node, wherein the determining comprises simulating a queue based on at least the simulated background traffic and the foreground traffic received at the node, wherein the determining further comprises: computing a rate of growth of the simulated queue based on at least the simulated background traffic and an output link capacity associated with the node; and calculating a length of the simulated queue based on at least the computed rate of growth and lengths of the foreground data packets; and (c) making a forwarding decision, at the node, with respect to the foreground traffic, depending on the calculated length of the simulated queue, and wherein the forwarding decision includes one of: (i) discarding a foreground data packet; or scheduling the foreground data packet for forwarding to the connection to the client via an egress media interface.
 27. The computer readable medium of claim 26, wherein the computing comprises computing the rate of growth of the simulated queue according to: a[k]=R[k] −(1/8)*C, wherein: a[k] represents the rate of growth of the simulated queue (in bytes per second) for the duration of a kth timeslot; R[k] represents a background traffic volume (in bytes per second) generated during the kth time slot; and C represents the output link capacity of the node (in bits per second).
 28. The computer readable medium of claim 26, wherein the forwarding decision includes one of: discarding the foreground data packet, forwarding the foreground data packet to the server, or scheduling the foreground data packet for forwarding to the server. 